The invention relates to the field of adhesives. More specifically, the invention relates to radiation curable adhesives based on epoxy-functional block copolymers.
Radiation curable adhesives which cure by free radical mechanisms have gained wide acceptance in a number of product assembly applications. Cationic curing systems enjoy several advantages over free radical systems. First, cure is not instantaneous and thus the possibility exists to irradiate the sample and then make the bond. This feature obviates the need for one substrate being radiation transparent, which many are not. Secondly, since there is no termination mechanism other than neutralization of the active acid catalyst by impurities, the cure proceeds into the xe2x80x9cdarkxe2x80x9d (i.e. non-irradiated) regions of the sample. Thus, if impurities are minimized the entire adhesive sample will eventually cure as long as one part of it is irradiated.
Most cationic systems use cycloaliphatic epoxide monomers or vinyl ethers. Kraton Polymers recently developed a system based on epoxidized block copolymers, with the epoxy groups residing almost exclusively in the polyisoprene blocks. These systems enabled highly flexible and even pressure sensitive radiation cured adhesives based on cationic cure chemistry. However, the systems have been hindered by high costs, primarily because of the use of expensive mono-ol liquids that have been used in place of mineral oils to maximize pressure sensitive adhesive properties, and also by the tendency for preferred photoinitiators (which are present as finely dispersed particles) to settle over time in the liquid or xe2x80x9cwarm meltxe2x80x9d adhesives.
A need remains for radiation curable adhesives that may be formulated at a lower cost and that avoid certain problems previously encountered in the art. The current invention addresses this need.
It has now been discovered that an improved balance of properties can be obtained by combining epoxidized block copolymers with other saturated block copolymers containing a hard block and/or by adding lower levels of rosin derived alcohols. Hot melt adhesives with superior pressure sensitive adhesive properties have been obtained by this method. In these solid adhesives settling of the dispersed photoinitiator particles is effectively eliminated, thus ensuring a long shelf life.
The invention provides adhesive formulations based on epoxidized block copolymers and contain a rosin-derived alcohol or a saturated block copolymer which are particularly advantageous for use as a labeling adhesive, i.e., for applying a label to a container such as, for example, a bottle, and for use in the manufacture of industrial tapes.
One aspect of the invention is directed to a radiation curable adhesive comprising an epoxidized block copolymer, a saturated block copolymer and/or a rosin derived alcohol, and a photoinitiator. Preferred saturated block copolymers are styrenic block copolymers. The adhesives of the invention may optionally comprise a hydrocarbon resin, a rosin and/or rosin ester, and an oil.
Preferred adhesives will typically comprise from about 10 to about 60% by weight of at least one epoxidized block copolymer, from about 3% to about 30% by weight of at least one saturated block copolymer and/or from about 3% to about 20% of a rosin derived mono-ol, from about 20% to about 70% of at least one hydrocarbon resin, up to about 25% of a rosin and/or rosin ester, up to about 0 to about 30% of a mineral oil, and an effective amount of a photoinitiator.
Another aspect of the invention is directed to an article of manufacture comprising the adhesive of the invention. In one embodiment the article is a labeled container. In another embodiment the article is an adhesive tape.